PHP Respect Validation password confirmation

Question

I used slim 3 framework with davidepastore/slim-validation to validate my data.

davidepastore/slim-validation itself uses Respect validation rules.

I used validation in middleware (in router file):

use DavidePastore\Slim\Validation\Validation;
use app\Controllers\RegisterController;
.
.
.

$app->post("register" ,[RegisterController::class , "register"])->add(new Validation(RegisterController::RegisterValidate(),$translator));

RegisterController::RegisterValidate() return some rules like :

[
      "password"              => v::length(6, 18) ,
      "password_confirmation" => v::key('password_confirmation', v::equals(v::key('password'))),
]

But it did not work. Always return error. Error message is "Key {{name}} must be present".

I used the v::keyValue('password_confirmation', 'equals', 'password') but it made no difference and returned the same error. Am I using the wrong method? Is it possible to reach the desired answer from this method? Or I have to change my method.

My request (JSON):

{
    "username":"[email protected]",
    .
    .
    .
    "password":"123654s",
    "password_confirmation":"123654s",
    .
    .
    .
    "name" : "Milito"
}

And I used POST HTTP method